During casting processes, molten metal flows from a first vessel into a second vessel or mold. For example, a common practice in the continuous casting of steel is to transfer the molten metal from a ladle vessel into a tundish vessel and from the tundish vessel into a mold or molds. A stream of molten metal typically issues from a nozzle, tube or shroud attached to the bottom of the ladle, and enters the tundish as a downwardly falling stream. The metal typically leaves the tundish as one or more exiting streams that flow through outlets in the bottom of the tundish.
Water modeling is an accepted method of simulating the flow of molten metal, and has been used to examine the flow of a stream of molten steel from a ladle into a tundish. Water modeling has shown that an incoming ladle stream is deflected from the tundish floor toward the surface of the molten steel. The deflected stream can surge upwards and generate excessive turbulence at the surface of the molten steel. Structural barriers, such as tundish side and end walls, can exacerbate turbulence. Excessive turbulence can disrupt the protective flux cover on the surface and incorporate flux particles in the molten steel. The resultant exposure to air can oxidize the steel. Flux particles can create inclusions in the solidified steel. Both factors negatively impact the final product.
Tundish impact pads are used to protect a tundish lining-from erosion by the ladle stream, but they are also used to control the deflected stream, turbulence and the flow of molten metal in a tundish. An impact pad is positioned on the bottom of the tundish to receive the incoming ladle stream. The impact pad includes an upper surface that is resistant to the impact force and erosive influence of the incoming stream of molten metal. When erosion of the impact pad does occur, the impact pad is more easily replaced than the tundish lining. The upper surface of the impact pad will generally be larger than the cross-section or diameter of the incoming steam to accommodate lateral and vertical movement of the tundish relative to the ladle.
A tundish impact pad of the prior art may simply consist of a flat slab of refractory material defining an upper surface. The impact pad may be placed upon or recessed in the bottom of the tundish. The pad will preferably be positioned beneath the ladle shroud such that the incoming stream will impact upon the upper surface of the pad. This configuration often does not attempt to control the deflected ladle stream.
Prior art also includes impact pads designed improve tundish flow behavior by redirecting the deflected stream. Prior art pads include shapes intended to alter the deflection pattern of the incoming stream and the overall flow behavior in a tundish bath so as to reduce splashing and turbulence in a tundish. U.S. Pat. No. 5,072,916 to Soofi teaches an impact pad with a wavy upper surface and wavy sidewalls that redirects and decelerates the reflected flow while reducing splashing, agitation, and turbulence of the flow. U.S. Pat. No. 5,358,551 to Saylor describes an impact pad with an endless sidewall completely around the periphery of the upper surface of the pad thereby defining an impact pad with an interior space. The endless sidewall includes an undercut that reverses the flow upwardly and inwardly.
US Re. 35,685 to Schmidt et al teaches an impact pad comprising an outer sidewall with an undercut surface that redirects and reverses the flow back on the incoming stream. Unlike Saylor, this sidewall is not described as endless, that is, the sidewall does not wrap entirely around the periphery of the upper surface. A primary portion of the flow exits along the bottom of the tundish toward the tundish opening, and is not directed upwardly toward the metal surface.
U.S. Pat. No. 4,776,570 to Vo Thanh et al. teaches a ladle stream breaker, which consists of a closed box having a top wall and lateral walls. The top wall contains an opening into which the lower end of the ladle shroud is fitted so that the incoming stream enters the box. Lateral walls have a plurality of simple, straight holes that allow the molten metal to exit the box as a plurality of low energy sub-streams. Without the top wall, the molten metal has no incentive to exit the holes and would likely exit out the top of the stream breaker. The stream breaker is described as inhibiting slag entrainment and allowing better inclusion separation. Unlike an impact pad, the stream breaker is a closed box having a top wall. Further, the stream breaker is fixed to the ladle shroud and impedes the relative lateral movement between the ladle and the tundish. Freedom to move the ladle shroud relative to the tundish is very advantageous, and arguably essential, to casting operations. The present inventors are aware of no use of ladle stream breakers.
Prior art impact pads do not adequately control the flow of molten metal in a tundish. Flat impact pads can exhibit excessive splash resulting in surface turbulence and oxidation of the metal. Slag entrainment can occur as the result of the strong upward flow components near the tundish walls and downward drag around the incoming stream. This behavior may result in dirtier steel and generally reduced metal quality. Shaped impact pads primarily redirect the flow upward toward the top surface of the tundish. The redirection of flow toward the bath surface may disturb this surface resulting in turbulence and liquid metal contamination by slag and gas. Also the overall flow pattern, developed in the tundish bath by prior art pads with an endless sidewall, does not provide the best opportunity for non-metallic floatation or for reducing the mixing between metal chemistries during chemical transition. Impact pads of the prior art with a sidewall that is not endless primarily redirect the flow outward near the bottom of the tundish. This redirection is beneficial in reducing surface disturbance does not provide the best conditions for non-metallic floatation or for reducing the mixing between metal chemistries.
Prior art impact pads with or without sidewalls, do not redirect flow in such a way that the flow is divided and distributed in a controlled manner between both the upward and outward directions, thereby reducing surface turbulence and simultaneously permitting separation of inclusions.